The negative effects of oxidizing conditions on vulcanizates obtained from polymers having carbon-carbon double bond unsaturation have long been a problem, particularly in applications where the vulcanizates are exposed to elevated temperatures for extended periods of time. A variety of approaches have been developed in the art in an attempt to solve this problem.
It is known that the carbon-carbon double bonds of such polymers activate the vulcanizate to oxidative attack. One solution to the problem of oxidative attack is to use polymers with few or no carbon-carbon double bonds. Examples of such polymers include butyl rubber (copolymers of isobutylene and isoprene) which typically contain from about 0.5 to about 3.0 mole percent of carbon-carbon double bond unsaturation, and ethylene-propylene copolymers which contain no such unsaturation.
Certain applications, such as the various hoses and seals in the engine compartment of automobiles, require vulcanized polymers with a combination of oil resistance, and resistance to oxidative attack in air at elevated temperatures for extended periods of time. Vulcanizates of copolymers of conjugated dienes and α,β-unsaturated nitrites, such as acrylonitrile-butadiene copolymers, commonly known as “nitrile rubbers” or “NBR”, are well known for their oil resistance. However, they contain carbon-carbon double bond unsaturation and therefore are susceptible to oxidative attack unless subjected to special compounding procedures for the production of oxidation resistant vulcanizates.
In order to reduce the amount of carbon-carbon double bond unsaturation in NBR and yet retain the copolymer's oil resistance which is thought to be provided by the nitrile functional groups in the copolymer, methods have been developed to selectively hydrogenate the carbon-carbon double bond unsaturation of NBR without hydrogenating the nitrile groups to produce hydrogenated NBR or HNBR.
See for example, GB-A-1,558,491, the contents of which are hereby incorporated by reference. A further review by Tan et al. Rubber Chem. Tech. (2005), 78(3), 489-515 gives a comprehensive overview of this technical field.
While the development of HNBR represents a significant advance in the art, there is still room for improvement.
In U.S. Pat. No. 6,214,922 it is disclosed that the usage of a combination of antioxidants together with bases significantly improves the hot air aging characteristics of hydrogenated nitrite vulcanizates. One preferred combination includes the usage of sodium carbonate which provides good results.
In US 2002/072557 A1 it is further disclosed to use anti-aging agents for organic polymers based on salts of sterically hindered phenolic compounds with at least two phenolic OH groups.
In WO-A-2007/072900 it has been shown that compression set values of a nitrite rubber may be improved by using an aromatic secondary amine antiaging agent and a polyamine crosslinking agent together with a nitrite rubber containing α,β-ethylenically unsaturated dicarboxylic acid monoester monomer units.
In JP 2008-056793 A a crosslinkable nitrite rubber composition having an improved tensile stress and scorching stability is disclosed containing a crosslinking agent and a highly saturated nitrite rubber having α,β-ethylenically unsaturated nitrite monomeric units and, α,β-ethylenically unsaturated dicarboxylic acid monoester monomeric units having one carboxyl group on each of the two carbon atoms forming the α,β-ethylenically unsaturated bond, and wherein the time tc(90) corresponding to 90% vulcanization is at least five minutes. The α,β-ethylenically unsaturated dicarboxylic acid monoester monomer is preferably a maleic acid mono n-butyl ester monomeric unit or a fumaric acid mono-n-butyl ester unit. As crosslinking agent contained in the crosslinkable nitrite rubber compositions polyamine compounds are preferred. The compositions may additionally contain a crosslinking promoter. DOTG (Di-o-tolyl guanidin) is the only crosslinking promoter mentioned.
EP 1 234 851 A1 discloses a crosslinkable rubber composition comprising a nitrite group-containing highly saturated copolymer rubber containing repeating units of an ethylenically unsaturated dicarboxylic acid monalkyl ester monomer, a polyamine crosslinking agent, and a basic crosslinking accelerator being a guanidine crosslinking accelerator such as tetramethylguanidine, tetraethylguanidine, diphenylguanidine, d-o-tolylguanidine, o-tolylbiguanidine and a di-o-tolylguadinine salt of dicathecolboric acid; or aldehydeamine crosslinking accelerators such as n-butylaldehydeaniline, acetaldehydeammonnia and hexamethylenetetramine. Of these, guanidine crosslinking accelerators are preferred. According to all examples in EP 1 234 851 A1 DOTG is used as crosslinking accelerator.
The latter two references show, that even though the vulcanizates mentioned in the prior art are very useful for improved long term compression set values, for an optimized crosslinking reaction a strong organic base as e.g. 3-di-o-tolyl guanidine (DOTG) is needed.
Caused by the toxicity of DOTG the use of a curing system containing this crosslinking accelerator is restricted in use for certain applications.
Hence, there was a need to develop and provide novel vulcanizable polymer compositions avoiding the use of toxic components and in particular toxic cross-linking accelerators and to further provide polymer vulcanizates based on such polymer compositions, which vulcanizates should show improved physical properties such as hot air aging and a low compression set and simultaneously still have a lower toxicity. It is yet another object of the present invention to provide a novel process for producing said novel polymer vulcanizates.